1. Field of the Invention
The present invention relates to a focus control device, in particular, a focus control device, which performs autofocus control by using a contrast mode, a method therefor, and a program which causes the focus control device to implement the corresponding method. Further, the invention also relates to a lens system corresponding to such a focus control device, a focus lens driving method for the lens system, and a program which causes the lens system to implement the corresponding method.
2. Description of the Related Art
To perform an autofocus control, a contrast mode is used. If the contrast of a captured image is high, this means that the blur in the captured image is low by the same amount, and thus it can be assumed that this state corresponds to an in-focus state. The contrast mode is an autofocus control mode based on such an assumption.
Specifically, in the contrast mode, the contrast of the captured image signal is measured (detected) while shifting a focus lens. Then, by shifting the focus lens to a position at which the measured contrast becomes the maximum, it is possible to obtain a state (in-focus state) in which a subject is brought into focus.
In a practical contrast mode, the contrast is not continuously detected from a detection range in which the focus lens is shifted, but a plurality of detected values is obtained by performing detection at a plurality of detection positions which are separated from each other with intervals. Then, from such detected values, a focus lens position at which the contrast becomes the maximum is obtained.
In such a manner, in the contrast mode, a predetermined interval (a detection interval) is provided for each detection position in the detection range. As the detection interval increases, the number of detection positions in the detection range decreases. Therefore, the time necessary to detect the entire detection range decreases. However, as the detection interval increases, the amount of change in contrast for each detection position becomes large. Hence, the accuracy in detection of the position, at which the contrast becomes the maximum, becomes low. In contrast, when the detection interval becomes narrow, the amount of change in contrast for each detection position becomes small. Thus, the accuracy in detection of the position, at which the contrast becomes the maximum, becomes high. However, the time necessary to detect the entire detection range increases, and as a result, the time necessary to obtain the in-focus state also increases.
Accordingly, in the related art, there are existing configurations in which, as a user presses the release button halfway, the detection interval is changed in accordance with the depth of field at the start timing of the autofocus control. That is, for example, when a subject is near, the depth of field is shallow. In this case, the detection interval is changed to be narrow (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-48126 (FIG. 15)). Thereby, it is possible to increase the accuracy of focusing even under situation in which focusing is difficult since the depth of field is small. On the other hand, when the depth of field is large, by setting a large detection interval, it is possible to shorten the time necessary to reach the in-focus state.